To control the reactivity of a nuclear core contained in the pressure vessel of a nuclear reactor, it is customary to use control rods which extend through the vessel into the core and which have inner ends provided with neutron-absorbing material, the rods being longitudinally moveable so that the material can be inserted more or less into the core. On the outside of the vessel the rods are provided with powered drives through which the control rods extend outwardly into pressure tubes into which the rods can be withdrawn and which, together with pressure-tight housings for the drives, confine the coolant pressure within the pressure vessel.
Such arrangements are normally vertical and in the case of conventional light-water reactors, the drives include holding latches which, when released, permit the rods to drop to scram the core. Such holding latches are of the ratchet and pawl type and do not by themselves hold the rods against upward motion withdrawing the neutron-absorbing material from the core.
If the integrity of the pressure confinement of the drives and rods should fail, the coolant pressure in the pressure vessel applies a force in a direction driving the rods away from the core, and if the drive mechanisms are unable to hold the rods, the neutron-absorbing material is withdrawn from the core so that the latter's reactivity is no longer under the control of that material. It is possible for the rods to be driven away from the core at high velocities.
A control rod can be locked against such inadvertent withdrawal, by using a screw and nut drive, but this does not permit rapid insertion of the rod. Therefore, it has become conventional to use a drive mechanism of the electromagnetically actuated, stepping type, with the holding latch of the ratchet and pawl type arranged to hold the rod upwardly against its weight during the stepping operation, the holding latch being inoperative to hold the rod against withdrawal or upward motion, the drive mechanism alone being relied on for this function.
This possibility of the control rod being suddenly driven away from the core by a pressure loss on the part of the drive's housing and the pressure tube extending from it, is very serious in the case of fast gas-cooled breeder reactors whose characteristics do not permit a control rod to be driven away from the core for even a brief time, in the event of trouble.